Short answer
Both are dongles, $79 against $60.
The FiiO KA3 reaches 100 dB on 565 of the 567 models here, the Kiwi Ears Allegro Pro on 554.
| Specification | FiiO KA3 | Kiwi Ears Allegro Pro |
|---|---|---|
| Power into 32Ω | 130 mW | 68 mW |
| Power into 150Ω | 28 mW | 15 mW |
| Power into 300Ω | 14 mW | 7.3 mW |
| Power into 600Ω | 7.0 mW | 3.6 mW |
| Output impedance | 0.5 Ω | not published |
| Output noise | 1.7 µV | not published |
| Price | $79 | $60 |
| Headphones it reaches 100 dB on out of 567 | 565 | 554 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO KA3 | Kiwi Ears Allegro Pro |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 240 mW | 170 mW |
| Power into 300Ω | 56 mW | 18 mW derived |
| Output impedance | 0.7 Ω | not published |
| Output noise | 2.9 µV | not published |
| Headphones it reaches 100 dB on out of 567 | 565 | 563 |
Through the balanced sockets the FiiO KA3 reaches 100 dB on 565 of the 567 models here and the Kiwi Ears Allegro Pro on 563. Derived figures are worked out from the loads the maker did publish.
| Load | FiiO KA3 | Kiwi Ears Allegro Pro |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Published | Derived |
Both makers publish a power rating. FiiO KA3 publishes at 32 Ω, at 300 Ω and at 600 Ω; Kiwi Ears Allegro Pro at 32 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω and at 600 Ω only the FiiO KA3 figure is the maker’s own. On those rows the derived number is the one to doubt first.
11 headphones reach 100 dB on the FiiO KA3 but not on the Kiwi Ears Allegro Pro — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 8 more.
554 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Campfire Audio Andromeda 2020: FiiO KA3 has +12 dB of headroom there against +7 dB. 5 dB is about 3.5 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Kiwi Ears Allegro Pro because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO KA3 headroom | Kiwi Ears Allegro Pro headroom | FiiO KA3 hiss | Kiwi Ears Allegro Pro hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +32 dB | +27 dB | 19 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +12 dB | +7 dB | -1 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +19 dB | +16 dB | -3 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +11 dB | -7 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +12 dB | +8 dB | -8 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +8 dB | +5 dB | -14 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +0 dB | -18 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +1 dB | -1 dB | -20 dB | not published |
The FiiO KA3 costs $19 more than the Kiwi Ears Allegro Pro. Out of the 567 models here, that difference buys 11 more models driven to 100 dB.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.
Each in full: FiiO KA3 and Kiwi Ears Allegro Pro.